Bi-pyridyl-rumetal complexes

ABSTRACT

A compound of formula (I): (X) n  RuLL 1  where n is 1 or 2, preferably 2, and in which Ru is ruthenium; each X independently is selected from Cl, SCN, H 2  O, Br, I, CN and SeCN and L is a ligand of formulae (a) to (g) and L 1  is selected from a ligand of formulae (a) to (c) where each R independently is selected from OH, hydrogen, C 1-20  alkyl, --OR a  or --N(R a ) 2  and each R a  independently is hydrogen or C 1-4  alkyl.

This application is a 371 application of PCT/EP93/02221.

The invention relates to new transition metal dyestuffs and to their usein photovoltaic cells. These dyes can be coated on titanium dioxidefilms rendering the device effective in the conversion of visible lightto electric energy.

Titanium dioxide films (layers) are known for their semiconductiveproperties and this property renders them useful for photovoltaic cells.However titanium dioxide has a large band gap and therefore it does notabsorb light in the visible region of the spectrum. For solarapplications it is important that the titanium dioxide film be coatedwith a photosensitizer which harvests light in the wavelength domainwhere the sun emits light, i.e. between 300 and 2000 nm. Thermodynamicconsiderations show that conversion of solar energy into electricity isachieved in the most efficient fashion when all the emitted photons withwavelengths below 820 nm are absorbed by the photosensitizer. Theoptimal dye for solar conversion should therefore have an absorptiononset around 800 nm and the absorption spectrum should be such that itcovers the whole visible domain.

A second requirement for efficient solar light energy conversion is thatthe dyestuff after having absorbed light and thereby acquired anenergy-rich state is able to inject with practically unit quantum yield,an electron in the conduction band of the titanium dioxide film. Thisrequires that the dyestuff is attached to the surface of the titaniumdioxide through suitable interlocking groups. The function of theinterlocking group is to provide electronic coupling between thechromophoric group of the dyestuff and the conduction band of thesemiconductor. This type of electronic coupling is required tofacilitate electron transfer between the excited state of the dyestuffband the conduction band. Suitable interlocking groups are p-conductingsubstituents such as carboxylate groups, cyano groups, phosphate groupsor chelating groups with p-conducting character such as oximes,dioximes, hydroxy quinolines, salicylates and alpha keto enolates. Theelectrons, photoinjected by the dyestuff, generate electrical current inthe external circuit when the photovoltaic cell is operated.

Accordingly a new series of dyes has been developed to act as aphotosensitizer.

According to the invention, there is provided a compound of formula I

    (X).sub.n Ru LL.sub.1                                      ( 1)

where

n is 1 or 2, preferably 2,

and in which

Ru is ruthenium;

each X independently is selected from Cl, SCN, H₂ O, Br, l, CN, --NCOand SeCN; and

L is a ligand of the formula ##STR1## and L, is selected from a ligandof formula a) to g) ##STR2## where each R independently is selected fromOH, hydrogen, C₁₋₂₀ alkyl, --OR₂ or --N(R_(a))₂

and

each R_(a) independently is hydrogen or C₁₋₄ alkyl

Preferred groups of L, are those of formula a) b) c) and d).

Preferably X is X' where X' is Cl, CN, --NCO or --SCN.

Preferably L₁, is L₁ ', where L₁ ', is selected from a ligand of formulaa', b, and c' ##STR3## in which R' is OH, hydrogen or C₁₋₂₀ alkyl, (morepreferably R" where R" is hydrogen or C₁₋₂₀ alkyl).

More preferred compounds of formula I are of formula II

    (X').sub.2 RuL"L                                           (II)

in which

L" a group of formula a" or b ##STR4## L is as defined above: X' isselected from Cl, CN --NCO and SCN; and

R" is C₁₋₂₀ alkyl (preferably C₁₋₁₅ alkyl) or hydrogen

Most preferably L" is a ligand of the formula b) ##STR5## Especiallypreferred is cis and trans (preferablycis)-dithiocyanato-bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium (II).

Preferably in formulae a, a' and a", the group R and R' respectively arelocated in the para position to the N atom in the 4-position.

According to the invention there is provided a photovoltaic cellcomprising:

an electrically conductive layer (preferably light transmitting)deposited on a support (preferably glass plate a transparent polymersheet or a metal surface) to which one or more (preferably porous highsurface area) titanium dioxide layers have been applied, characterizedby applying a compound of formula I defined above (as a photosensitizer)to the TiO₂ layer.

Still further according to the invention, there is provided aphotovoltaic cell comprising:

i) two electrodes, at least one of which is transparent and has avisible light transmittance of at least 60%, the plates being arrangedso as to define a receptacle between them, in which receptacle anelectrolyte is located, one of the electrodes having a film of titaniumdioxide (preferably high surface area) the film being coated with aphotosensitizer; and

ii) means for permitting the passage of an electrical current generatedby the cell;

characterized in that the photosensitizer is a compound of formula Idefined above.

Preferably a photovoltaic cell according to the invention comprises,

i) an electrically conductive first plate to which a film of titaniumdioxide is applied, (the film preferably having a thickness of 0.1-50microns) and the film being coated with a photosensitizer; and

ii) a conductive second plate with no TiO₂ film separated from the firstplate by a thin layer of electrolyte, whereby the visible lighttransmittance of at least one of the plates is at least 60%; (preferablyalso for solar light);

characterized in that the photosensitizer is a compound of formula Idefined above.

The second plate (also known as "the counterelectrode") may be coatedwith a thin layer (preferably up to 10 microns thickness) of anelectrocatalyst. The role of the electrocatalyst is to facilitate thetransfer of electrons from the counterelectrode to the electrolyte. Afurther possible modification of the counterelectrode is to make itreflective to light that has first passed through the electrolyte andthe first plate.

Further the outside of the glass plates may be coated with plastics likePS, PMMA or preferably PC to protect the TiO₂ layer, the dyestuff andthe electrolyte against UV-light to give long term stability.

Preferably the electrolyte contains a redox system (charge transferrelay). Preferably such systems include iodine/iodide solutions,bromine/bromide solutions, hydroquinone solutions or solutions oftransition metal complexes transferring a nonbonding electron. Thecharge transfer relays present in the electrolyte transport electriccharge from one electrode to the other. They act as pure mediators andundergo no chemical alteration during the operation of the cell. It ispreferable that the electrolytes in a photovoltaic cell according to theinvention are dissolved in an organic medium so that the dyes applied tothe titanium dioxide surface are insoluble therein. This has theadvantage that the cell has a long-term stability.

Preferred organic solvents for the electrolyte include but are notlimited to water, alcohols and mixtures thereof, non-volatile solventssuch as 3-methyl(-2-oxazolidinone (NMO), 1,3-dimethyl-2-imidazolidinone(DMEN), propylene carbonate, ethylene carbonate and methylpyrrolidinone, mixtures of non-volatile solvents with viscosity reducingsolvents such as acetonitrile, ethylacetate or tetrahydrofuran.Additional solvents are dimethylsulfoxide or dichloroethane. Wheremiscible, mixtures of any of the above may be used.

Preferably the titanium dioxide films have a roughness factor greaterthan one, the roughness factor being defined as the ratio of true toapparent surface area. More preferably the roughness factor is 10-1000,most preferably 50-200. Preferably the titanium dioxide layers are builtup on the surface of the conductive layer using the on of two methods.One, the sol-gel method is described in "Stalder and Augustynski, J.Electrochem. Soc. 1979, 126:2007" and in Application Example A. Theother, the "colloidal method" is described in Application Examples B andD.

In the sol gel method it is preferable that only the last three, thelast two or just the very top layer of the titanium dioxide is dopedwith a divalent or trivalent metal in an amount of not more than 15%doping by weight. However, the deposition of the pure dopant in form ofa very thin top oxide layer can also be advantageous. In the lattercases a blocking layer is formed which impedes leakage current at thesemiconductor-electrolyte junction. All of the TiO₂ layers are formed bythe sol gel process method described in Application Example A.Preferably the number of TiO₂ layers deposited is 10-11. Preferably thetotal thickness of the TiO₂ film is from 5 to 50 microns (morepreferably 10-20 microns).

The glass or polymer plate which is used for the transparent plate ofthe cell according to the invention is any transparent glass or polymeronto which a light transmitting electrically conductive layer has beendeposited, such that the plate preferably has a visible lighttransmittance of 60-99%, more preferably 85-95%. Preferably thetransparent conductive layer has a surface resistance of less than 10ohms per square cms, preferably from 1 to 10 ohms per square cm.Preferably the transparent conductive layer used in a photovoltaic cellaccording to the invention is made of tin dioxide doped with ca. 0.8atom percent of fluorine and this layer is deposited on a transparentsubstrate made of low-cost soda lime float glass. This type ofconducting glass can be obtained from Asahi Glass Company, Ltd. Tokyo,Japan. under the brand name of TCO glass. The transparent conductivelayer can also be made of indium oxide doped with up to 5% tin oxide,deposited on a glass substrate. This is available from Baizers under thebrand name of ITO glass.

The photosensitising layer may be produced by applying to the TiO₂ layera dye according to the invention defined below.

Cis-(X)₂ bis(2,2'-bipyridyl-4,4'-dicarboxylate)-ruthenium(II)complexes(X=Cl,Br,CN and SCN) act as charge tranfsersensitizers for nanostructured TiO₂ films (thickness 8-12 μm) of veryhigh internal surface acrea (roughness factor ca. 1000), prepared bysintering of 15-30 nm-sized colloidal titania particles on a conductingglass support. The performance of cis-dithiocyanatobis(2.2'-bipyridyl-4,4'-dicarboxylate)-ruthenium(II) is especially good.Nano-structured TiO₂ films coated with a mono layer of 1 μ harvestvisible light very efficiently, their absorption threshold being around800 nm. Conversion of incident photons into electric current is nearlyquantitative over a large spectral range. These films can beincorporated in a thin layer regenerative solar cell equipped with alight-reflecting counterelectrode. The open circuit voltage can beincreased usefully by treating the dye-covered film with 4-tertbutylpyridine. The effect of temperature on the power output and longterm stability of the dye is also investigated. For the first time, adevice based on a simple molecular light absorber attains a conversionefficiency commensurate with that of conventional silicon basedphotovoltaic cells.

While tris(2,2'-bipyridyl)ruthenium(II) and it homologues have beenextensively investigated as redox sensitizers, very little is knownabout the excited state redox properties of thebis(2,2'-bipyridyl)ruthenium(II) analogues. The reason for this is thatthe excited state of these compounds is often too short-lived to allowfor accurence of homogeneous bimolecular electron transfer reactions.However, heterogeneous charge transfer processes might still beinitiated with such sensitizers since they can take place over a veryshort time scale. Apart from their chemical stability and ease ofinterfacial charge exchange with semiconducting solids, it has beenfound that these complexes have a large visible light harvestingcapacity which is superior to that of the widely studied trisbipyridylRu(II) analogues, making them useful for use in a solar energyconversion device.

One aspect of the invention relates to Ru(II) complexes having thegeneral formula cis-(X)₂bis(2,2'-bipyridyl-4,4'-dicarboxylate)-ruthenium(II) complexes, where Xis Cl, Br, l, CN and SCN. Among these complexescis-dithiocyanatobis(2,2'-bipyridyl-4,4'-dicarboxylate)-ruthenium(II)displays outstanding properties as a charge transfer sensitizer. Itsbroad range of visible light absorption and relatively long-livedexcited state renders it an attractive sensitizer for homogeneous andheterogeneous redox reactions. In conjunction with the recentlydeveloped nanostructured colloidal TiO₂ films and the iodide/triiodideelectrolyte in a acetonitrile/3-methyl-2-oxazolidinone solvent mixture.this complex converts an appreciable amount of AM1.5 solar radiationinto electrical energy approaching the performance of polycrystallinesilicon photovoltaic cells.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a photovoltaic device based on the sensitization of analuminum-doped titanium dioxide film supported on conducting glass.

The invention will now be illustrated by the following Examples.

EXAMPLE 1

Preparation of Ru-bis(4,4'-COOH-2,2'-bipyridyl)-dichloride dihydrate, acompound of formula 1 a ##STR6##

Cis--Ru(II)L₂ Cl₂ --2H₂ O is obtained by refluxing under Ar 60 mg (0.229mmol) of RuCl₃.3H₂ O(Fluka 38-40% Ru) and 113 mg (0.463 mmol) of ligandL=2,2'-bipyridyl-4,4'-dicarboxylic acid (Aldrich) in 20mL of DMF for 8hours. After cooling, traces of RuL₃ are filtered. Most of the DMFsolvent is evaporated under vacuum and cis-Ru(II)L₂ Cl₂ is precipitatedwith acetone. The crystals are filtered off and dried in vacuum.Elemental analysis corresponds to C₂₄ H₁₆ N₄ O₈ Cl₂ Ru.2H₂ O

EXAMPLE 2

Preparation of cis-dithiocyanato-N-bis(2,2'-bipyridyl-4,4'di-carboxylicadd) ruthenium(II), dihydrate [RuL₂ (NCS)₂ ]2H₂ O, a compound of formula2a ##STR7## 283 mg of (0.428 mmol) RuL₂ Cl₂ are dissolved in 30 ml ofDMF, under reduced light. 350mg (4.25 mmol) of sodium thiocyanate areseparately dissolved in 2 ml of water and subsequently added to theabove solution. Then the reaction mixture is heated to reflux for 6hours under a nitrogen atmosphere, while maintaining magnetic stirring.After this time, the reaction mixture is allowed to cool and then thesolvent is removed on a rotary evaporator. The resulting solid isdissolved in water and filtered through a sintered glass crucible. ThepH of this filtrate is lowered to 2.5 by adding dilute HClO₄ or CF₃ SO₃H and placed in a refridgerator over night. After allowing it to reachroom temperature, the micro crystalline solid is isolated by suctionfiltration, washed well with water acidified with HClO₄ to pH3 andacetone-ether mixture (1:10) followed by anhydrous diethylether and airdried for an hour. The yield is 80%. Elemental analysis for C₂₆ H₁₆ N₆S.sub. 2 O₈ Ru.2H₂ O gives (calculated values in brackets):C41.65(42.1); H2.86 (2.272); N11.26(11.34).

Further structural characterization of 2 a can be carried out by IRspectroscopy using a DA3.26 (BOMEM Inc., Quebec, Canada) FTIR or aPerkin-Elmer 6811 instrument. The high resolution spectrum exhibits adoublet with peaks at 2126 and 2093 cm⁻¹ which is characteristic Of thecis-configuration of the two thiocyanate ligands. Furthermore, theN-coordination of the thiocynate group is confirmed by the presence ofthe v(C=S) resonance at 770 cm⁻¹.

EXAMPLE 3

a) Synthesis of RuCl₂ (DMSO)₄ where the DMSO=dimethylsulphoxide.

1 g of Ruthenium trichloride trihydrate is refluxed in 5ml ofdimethylsulphoxide for 1 hour. After this period of time, the additionof acetone gives a yellow precipitate.

The yellow complex which separates is filtered off, washed with acetoneand diethylether. The yield is about 80%

b) Synthesis of RuCl₂ (L) (DMSO)₂ where L is4,4'dimethyl-2,2'bipyridine.

1 equivalent of RuCl₂ (DMSO)₄, prepared above and 1 equivalent of4,4'dimethyl-2,2'bipyridine are refluxed in chloroform for 1 hours. Thesolution is cooled and the solvent is removed on a rotary evaporator.The resultant solid is dissolved in acetone and filtered. Addition ofdiethylether to the filtrate gives a yellow precipitate. This iscollected on glass frit, washed with ether and vacuum dried (givingabout 80% yield).

c) Synthesis of RuLL, Cl₂ where L is 4,4'-dimethyl-2,2'-bipyridine andL₁ is 4,4'carboxy-2,2'bipyridine (hereinafter the complex of Example3c).

1 equivalent of RuCl₂ L (DMSO)₂, prepared above and 1 equivalent of4,4'dicarboxy-2,2'-bipyridine are placed in dimthylformamide and themixture is refluxed for 3-4 hours in the dark. After this period, thereaction mixture is filtered through sintered glass crucible. Thefiltrate is evaporated and the resultant solid is washed with a 1:1mixture of acetone/ether and then washed with diethylether and vacuumdried.

EXAMPLE 4

Synthesis of RuLL,(SCN)₂ where L is 4,4'-dimethyl-2,2'-bipyridine and L,is 4,4'carboxy-2,2'bipyridine (hereinafter the complex of Example 4):

This is prepared by an analogous procedure to that described in Example2 from appropriate reactants.

EXAMPLE 5

a) Synthesis of RuCl₂ (L) (DMSO)₂ where L is 2,2'-bipyridine. This isprepared by an analogous procedure to that described in Example 3b fromappropriate reactants

b) Synthesis of RuLL, Cl₂ where L is 2,2'-bipyridine and L₁ is4,4'carboxy-2,2'bipyridine. This is prepared by an analogous procedureto that described in 3c from appropriate reactants.

EXAMPLE 6

Synthesis of RuLL₁ (SCN)₂ where L is 2,2'-bipyridine and L, is4,4'carboxy-2,2'bipyridine.

This is prepared by an analogous procedure to that described in example2 from appropriate reactants.

EXAMPLE 7

a) Synthesis of RuCl₂ (L) (DMSO)₂ where L is 4,4'-di-C₁₃alkyl-2,2'-bipyridine.

This is prepared by an analogous procedure to that described in example3b from appropriate reactants.

b) Synthesis of RuLL₁ Cl₂ where L is 4,4'-di-C₁₃ alkyl-2,2'-bipyridineand L, is 4,4'-dicarboxy-2,2'bipyridine.

This is prepared by an analogous procedure to that described in example3c from appropriate reactants

EXAMPLE 8

Synthesis of RuLL₁ (NCS)₂ where L is 4,4'-di-C₁₃ alkyl-2,2'-bipyridineand L₁ is 4,4'-dicarboxy-2,2'bipyridine.

This is prepared by an analogous procedure to that described in example2 from appropriate reactants

EXAMPLE 9

Cis-Dicyanobis(2,2'-bipyridyl-4,4'-dicarboxylic acid)ruthenium(ll)trihydrate is synthesized by dissolving 235 mg (3.6 mmol)of KCN in 10 ml of H₂ O and transferring it into a three-neck flaskcontaining 30 ml of DMF. Subsequently, 500 mg (0.76 mmol) of RuL₂Cl₂.2H₂ O of Example 1 are introduced into the above solution. Thereaction mixture is then heated to reflux under nitrogen for 5 hours.During this period, the initially violet color of the solution changesto orange. After this time, the reaction mixture is allowed to cool andthen filtered through a glass frit. The filtrate is evaporated todryness on a rotary evaporator and the resulting solid is dissolved inH₂ O at pH 10. Upon addition of dilute HClO₄ or CF₃ SO₃ H to the abovesolution, most of the complex precipitates as a neutral salt at pH 2-3.The orange solid is isolated by suction filtration, washed well with H₂O, ethyl alcohol, followed by anhydrous diethylether and air dried foran hour. The yield is 70%.

EXAMPLE 10

Synthesis of [Ru L (PPh₃)₂ (Cl)₂ ] where L is selected from the ligands4,4-dicarboxy-2,2-bipyridine (the ligand of formula b).

This is prepared by analogous procedure to that described in example 3bfrom appropriate reactants, using low or high boiling organic solvents.Most preferable solvent is acetone.

Ph=phenyl

EXAMPLE 11

Synthesis of [RuLL₁ (Cl)₂ ] where L is selected from the ligand offormula b and L₁ is 4,4 COOH-2,2-bipyridine.

This is prepared by analogous procedure to that described in example 3cfrom appropriate reactants, using low or high boiling organic solvents.Most preferably solvent is N,N-dimethylformamide.

EXAMPLE 12

Synthesis of [RuLL₁ (NCS)₂ ] where L is selected from the ligand offormula b) and L₁ is 4,4 COOH2,2-bipyridine. This is prepared by ananalogous procedure to that described in example 2 from appropriatereactants.

EXAMPLE 13

a) Synthesis of [Ru(CO)₂ (Cl)_(n) where n is 2 or 3.

Ruthenium trichloride (1 gr) is refluxed with 90% formic acid until thesolution changes from green to yellow over 3-4 hours. The solution isthen filtered and evapored on the steam bath to give the bright a yellowproduct, having yield of 90%.

b) Synthesis of [Ru L (CO)₂ (Cl)₂ ] where L is selected from the ligandof formula (b).

This is prepared by analogous procedure to that described in example 3busing low or high boiling solvents. Most preferable solvent is EtOH orMeOH. When appropriate reactants i.e. the compound of example 13a andthe ligand b) are heated at reflux in MeOH for 15 to 60 minutes, theyellow solid precipitates and is isolated as a fine yellow crystalshaving yield of 90%.

c) Synthesis of [RuLL₁ (Cl)₂ ] where L and L₁ are4,4-COOH-2,2-bipyridine.

This is prepared by analogous procedure to that described in example 3cfrom appropriate reactants, i.e. the compound of example 13b and4,4-COOH-2,2-bipyridine using low or high boiling organic solvents. Mostpreferable solvent is N,N-dimethylformamide.

EXAMPLE 14

Synthesis of [RuLL₁ (NCS)₂ ] where L and L₁ are 4,4-COOH-2,2-bipyridine.

This is prepared by an analogous procedure to that described in example2 from appropriate reactants i.e. the compound of example 13c and NaNCS.

EXAMPLE 15

Synthesis of [RuLL₁ Cl₂ ] where L is 4,4 -COOH-2,2-bipyridine and L₁ isselected from the group of formula (c), where R is hydrogen.

EXAMPLE 16

Synthesis of [RuLL₁ Cl₂ ] where L is 4,4-COOH-2,2-bipyridine and L₁ isselected from the group of formula (d), where R is hydrogen.

EXAMPLE 17

Synthesis of [RuLL₁ Cl₂ ] where L is 4,4-COOH-2,2-bipyridine and L₁ isselected from the group of formula (e), where R is hydrogen.

These compounds i.e. example 15, 16 and 17 can be prepared by ananalogous procedure to that described in example 3c from appropriatereactants.

EXAMPLE 18

a) Synthesis of [RuL₁ (Cl)₃ ].

1 equivalent of Ruthenium trichloride and 1 equivalent of L₁ [where L₁is selected from the ligand of the formula (f) or (g) where R ishydrogen] are refluxed in EtOH for 30 to 60 minutes. After this period,the resulting solid was collected on G4 crucible and washed with EtOH.

b) Synthesis of [RuLL₁ Cl] where L is 4,4-COOH2,2-bipyridine and L₁ isselected from the group of formula (f) where R is hydrogen.

This is prepared by refluxing 1 equivalent of [RuL₁ (Cl)₃ ] and 1equivalent of L in DMF for 3 to 5 hours. After allowing to roomtemperature, the solvent DMF is evaporated and the resulted solid isdissolved in water. The pH of this solution is adjusted to 2.5 bydiluted HClO₄, which give a dense precipitate. This precipitate iscollected on crucible and washed with water acidified with HClO₄ to pH 3and acetone-ether mixture followed by anhydrous diethyl ether. Theproduct was air dried for 2 hours.

EXAMPLE 19

Synthesis of [RuLL₁ Cl] where L is 4,4-COOH-2,2-bipyridine and L₁ isselected from the group of formula (g) where R is hydrogen.

This is prepared by an analogous procedure to that described in example18b from appropriate reactants i.e. the compound of formula 18a and4,4-COOH-2,2-bipyridine.

Application Example A

A photovoltaic device shown in FIG. 1 and based on the sensitization ofan aluminum-doped titanium dioxide film supported on conducting glass isfabricated as follows:

A stock solution of organic titanium dioxide precursor is prepared bydissolving 21 mmol of freshly distilled TiCl₄ in 10 mL of absoluteethanol. TiCl₄ in ethanol solution gives titanium alkoxide spontaneouslywhich, on hydrolysis, gives TiO₂ The stock solution is then diluted withfurther absolute ethanol to give two solutions (solution A and solutionB having) titanium contents of 25 mg/ml (solution A) and 50 mg/ml(solution B). A third solution (C) is prepared from solution B byaddition of AlCl₃ to yield an aluminium content of 1.25 mg/ml. Aconducting glass sheet provided by Asahi Inc. Japan, surface area 10 cm²and having a visible light transmittance of at least 85% and a surfaceresistance smaller than 10 ohms per square cm is used as support for adeposited TiO₂ layer. Prior to use, the glass is cleaned with alcohol. Adroplet of solution A is spread over the surface of the conducting glassto produce a thin coating. Subsequently the layer is hydrolyzed at 28°C. for 30 minutes in a special chamber, where the humidity is kept at48% of the equilibrium saturation pressure of water. Thereafter, theelectrode is heated in air in a tubular oven kept at 450° C., preheatingit in the entrance of the oven for 5 minutes followed by 15 minutes ofheating in the interior. Three more layers are produced in the same way.Subsequently, 5 thicker layers are deposited by using solution B. Thesame procedure as for the first layers is applied. Finally, solution Cis used to deposit the last two layers containing the aluminum dopant.The heating of the last layer in the tubular oven is extended from 15 to30 minutes. The total thickness of the titanium dioxide film is between10 and 20 microns.

Prior to deposition of the dye, the film is subjected to a sinteringtreatment in highly purified 99.997% argon. A horizontal tubular ovencomposed of quartz tubes with suitable joints is employed. Afterinsertion of the glass sheet with the TiO₂ film, the tube is twiceevacuated and purged with argon. The glass sheet is then heated underargon flux at a flow rate of 2.5L/h and a temperature gradient of 500°C./h up to 550° C. at which temperature it maintained for 35 minutes.This treatment produces anatase films with a surface roughness factor of80-200.

After cooling under a continuous argon flow, the glass sheet isimmediately transferred to an alcoholic solution (oralcoho/dimethylsulfoxide mixture) of a chromophore. The chromophoreemployed is the complex of Example 2. Its concentration in absoluteethanol is 5×10⁻⁴ M. Prolonged exposure of the film to the open airprior to dye adsorption is avoided in order to prevent hydroxylation ofthe TiO₂ surface as the presence of hydroxyl groups at the electrodesurface interferes with dye uptake. The adsorption of chromophore fromthe ethanolic solution is allowed to continue for 30 minutes after whichtime the glass sheet is withdrawn and washed briefly with absoluteethanol. The TiO₂ layer on the sheet assumed a deep red colour owing tothe chromophore coating.

The photocurrent action spectrum obtained with such a film using aconventional three electrode electrochemical cell containing anethanolic solution of 0.5M Lil and 3×10⁻³ M iodine is shown in theattached figure together with the AM 1 spectral distribution of solarlight emission. The incident monochromatic photon to current conversionefficiency (IPCE) is plotted as a function of the excitation wavelength.This was derived from the equation: ##EQU1## From the overlap of thephotocurrent action spectrum with solar emission the overall efficiencyfor the conversion of solar light to electricity η is calculated fromthe formula

    η=12×OCV×FF(%)                             (2)

where OCV is the open circuit voltage and FF is the fill factor of thephotovoltaic cell.

For experimental verification of equation 2, a photovoltaic cell, shownin the drawing attached, is constructed, using the dye of Example 1(4)-loaded TiO₂ (5) film supported on a conducting glass (the workingelectrode) comprising a transparent conductive tin dioxide layer (6) anda glass substrate (7) as a photoanode. The cell has a sandwich-likeconfiguration, the working electrode (4-7) being separated from thecounter electrode (1,2) by a thin layer of electrolyte (3) having athickness of ca. 20 microns. The electrolyte used is an ethanolicsolution of 0.5M Lil and 3×10⁻³ M iodine. The electrolyte (3) iscontained in a small cylindrical reservoir (not shown) attached to theside of the cell from where capillary forces attract it to theinter-electrode space. The counter-electrode comprises the conductivetin dioxide layer (2) deposited on a glass substrate (1) made also ofAsahi conducting glass and is placed directly on top of the workingelectrode. A monomolecular transparent layer of platinum is deposited onto the conducting glass of the counter electrode (1,2) by electroplatingfrom an aqueous hexachloroplatinate solution. The role of the platinumis to enhance the electrochemical reduction of iodine at the counterelectrode. The transparent nature of the counterelectrode is anadvantage for photovoltaic applications since it allows the harvestingof light from both the forward and the backward direction. Experimentsare carried out with a high pressure Xenon lamp equipped withappropriate filters to simulate AM1 solar radiation. The intensity ofthe light is varied between 50 and 600 Watts per square meter and theopen circuit voltage is 660 and 800 mV, respectively at these twovoltages. The fill factor defined as the maximum electric power outputof the cell divided by the product of open circuit voltage and shortcircuit current is between 0.7 and 0.75 V. A single crystal silicon cellgave an open voltage of 550 mV at 600 W/m² incident light intensitywhich dropped to below 300 mV at 50 W/m². This clearly shows that thecell of the present invention has a higher open circuit voltage than thesilicon solar cell and that the open circuit voltage is less dependenton light intensity than that of the silicon cell. This constitutes asignificant advantage for the use of such a cell in indirect sunlight orcloudy weather conditions. The fill factor of the silicon cell iscomparable to that of the example. The overall solar light toelectricity conversion efficiency of the cell of the example is between5 and 6% in agreement with predictions of equation 2.

Application Example B

A transparent TiO₂ film from colloidal titanium dioxide particles whichare deposited on a conducting glass support and sintered to yield acoherent highly porous semiconducting film that is transparent and canbe used instead of the TiO₂ layer film in Application Example A.

Colloidal titanium oxide particle of approximately 10 nm are prepared byhydrolysis of titanium isopropoxide as follows:

125 ml of titanium isopropoxide is added to a solution of 0.1M nitricacid in 750 ml of water whilst stirring. A precipitate of amorphoustitanium dioxide is formed under these conditions. This is heated to 80°C. for approximately 8 hours, stirring vigorously, resulting inpeptisation of the precipitate and formation of a clear solution ofcolloidal anatase. The anatase structure of the titanium dioxideparticles is established by Raman spectroscopy. The sol is concentratedby evaporation of the solvent in vacuum at room temperature until aviscous liquid is obtained containing the colloidal particles. At thisstage the nonionic surfactant TRITON X-100 (40% weight of TiO₂) is addedto reduce cracking of the film when applied to a substrate.

The titanium dioxide films are formed by spin coating the concentratedsol on to a conducting glass substrate. Usually it is sufficient toapply 6 to 10 layers in order to obtain semiconductor membranes ofsufficient surface area to give excellent visible light harvestingefficiencies after deposition of a monolayer of the sensitizer.

Low resolution electron microscopy confirms the presence of the threelayer structure, the lowest being the glass support followed by the 0.5micron thick fluorine-doped SnO₂ and the 2.7 micron thick titaniumdioxide layer. High resolution electron microscopy reveals the TiO₂ filmto be composed of a three dimensional network of interconnectedparticles having an average size of approximately 16 nm. Apparently,significant particle growth occurs during sintering.

The transparent TiO₂ films are tested in conjunction with a sensitizer,the dye of Example 2 regenerative cell for the generation of electricityfrom visible light. The results can be represented where thephotocurrent under simulated sunlight (intensity ca 30 W/m²) is plottedas a function of cell voltage.

Application Example C

A sheet of conducting glass (ASAHI) area resistance ca 10 Ohm/square cm)having a size of 2×9.6 cm² is coated with a colloidal titanium dioxidefilm according to the procedure of Example B. A total of 7 layers ofTiO₂ colloid are deposited successively by spin coating and the film issubjected each time to calcination at 500° C. for 30 minutes. 30% (w/w)of TRITON X 405 surfactant is added in order to avoid cracking of thefilm.

The final thickness of the titanium dioxide film is 5 microns asdetermined from the optical interference pattern. It is important tonote that the conducing glass sheet after deposition of the TiO₂ remainsclear and transparent to visible and near infrared light. Thetransmission spectrum recorded on a conventional spectrophotometer showsthat a fraction of more that 60% of the visible light in the wavelengthregion between 400 and 900 nm is transmitted through the film. AUV/visible absorption spectrum of the electrode can be obtained. Itexhibits a flat feature in the visible due to light absorption andscattering by the conducting glass and the 5 nm thick TiO₂ film. Thesteeply rising part of the absorption below 400 nm is due to the bandgap absorption of the TIO₂.

Immediately before coming with dyestuff, the film is fired for 1 hour at500° C. The coating of TiO₂ with dyestuff is performed by immersing theglass sheet for 16 hours in an ethanolic (or alcohol/diemthylsulfoixdemixture) solution containing the complex of Example 2. After coating,the glass sheet displays an intensive dark reel coloration. The opticalabsorption spectrum measured with a conventional UV/visiblespectrophotometer shows the absorbance in the vicinity of 500 nm toexceed the value of 2, indicating that in this wavelength range morethan 99% of the photons are absorbed by the dyestuff deposited on to thetitanium dioxide film. It is important to note that, due to the highconcentration of dyestuff, the porous film is capable of harvestingphotons over a very broad spectral range extending from 450 to 850 nm.

After dye deposition, the glass sheet is cut into two parts each havinga size of ca 9 cm². These sheets serve as working electrodes(photo-anodes) in the module whose assembly is described further below.

Transparent counterelectrodes are made of the same type of ASAHIconducting glass as the working electrodes. The counterelectrode is notcoated with TiO₂. Instead, the equivalent of 10 monolayers of Pt iselectrochemically deposited on to conducting glass. The transparentnature of the counterelectrode is not affected by the deposition of thePt its transmission in the visible and near infrared remaining greaterthat 60%. The Pt acts as an electrocatalyst, enhancing the rate ofreduction of the electron transfer mediator, i.e. triiodide, at thecounterelectrode. Two ca. 1 mm deep and 1.5 mm wide and 20 mm longindentations are engraved into the surface of the counterelectrode closeto the edges of the glass sheets. These serve as a reservoir for theelectrolyte.

The counter electrode is placed directly on top of the working electrodeto yield a sandwich-type configuration. After filling the reservoirswith electrolyte, the cell was sealed with epoxy resin. The wetting ofthe space between the two electrodes by the electrolyte occursspontaneously by capillary action. The electrolyte is a solution of 0.5Mtetrapropyl ammonium iodide and 0.02M iodine in ethanol.

Two cells are fabricated in this way, each having a surface area of ca9cm². Subsequently they are connected in series by electricallycontacting the photoanode of one cell to the cathode of the second cell.In this way a module is constructed, having a total surface area of18cm².

Application Example D

Nanostructured TiO₂ films are prepared by spreading a viscous dispersionof colloidal TiO₂ particles on a conducting glass support (Asahi TCOglass, fluorine-doped SnO₂ overlayer, transmission ca 85% in thevisible, sheet resistance 8 Ohm/square cm) and subsequent heating underair for 30 min at 30°-450° C. (preferably 450° C.). Two methods ofpreparation of colloidal TiO₂ dispersions are employed.

a) The procedure of Application Example C is repeated except thatautoclaving is performed at 230° to 240° C. instead of 500° C. Afterspreading the colloid on the conducting glass support and calcining, afew monolayers of TiO₂ are electrodeposited onto the colloidal TiO₂ filmfrom an aqueous Ti(III) solution followed by renewed annealing at 450°C. This treatment is found to improve significantly the short circuitphotocurrent as well as the open circuit voltage of the solar cell. Lowresolution electron microscopy confirms the presence of a three-layerstructure, the lowest being the glass support followed by the 0.7 μmthick fluorine-doped SnO₂ and the 10 μm thick colloidal TiO₂ film. Highresolution electro microscopy reveals the TiO₂ film to be composed of athree-dimensional network of interconnected particles having an averagesize of approximately 15 nm.

b) The second method for preparation of nanosturctured films (Method B)employed commercial TiO₂ (P25, Degussa AG, Germany, a mxiture of ca 30%rutile and 70% anatase). This is produced by flame hydrolysis of TiCl₄and consists of weakly aggregated particles. The BET surface areas is 55m² /g, corresponding to a mean particle size of about 25 nm. In order tobreak the aggregates into separate particles, the powder (12g) is groundin a porcelain mortar with a small amount of water (4 ml), containingacetylacetone (0.4 ml) to prevent reaggregation of the particles. Otherstabilizers such as acids, bases or TiO₂ chelating agents are found tobe suitable as well. After the powder has been dispersed by the highshear forces in the viscose paste it is diluted by slow addition ofwater (16 ml) under continued grinding. Finally, a detergent (0.2 mlTriton®X-100) is added to facilitate the spreading of the colloid on thesubstrate. The conducting TCO glass is covered on two parallel edgeswith adhesive tape (-40 μm thick) to control the thickness of the TiO₂film and to provide noncoated area for electrical contact. The colloid(5 μl/cm²) is applied to one of the free edges of the conducting glassand distributed with a glass rod sliding over the tape covered edges.After air drying, the electrode is fired for 30 minutes at 30°-550° C.(preferably 450°-550° C.) in air. The resulting film thickness is 12 μmbut can be varied by changing the colloid concentration or the adhesivetape thickness.

The performance of the film, as sensitized photoanode, is improved byfurther deposition of TiO₂ from aqueous TiCl₄ solution. A 2M TiCl₄ stocksolution is prepared at 0° C. to prevent precipitation of TiO₂ due tothe highly exothermic hydrolysis reaction. This stock solution isfreshly diluted with water to 0.2M TiCl₄ and applied onto the electrode(50 μl/cm₂). After standing overnight at room temperature in a closedchamber, the electrode is washed with distilled water. Immediatelybefore dipping into the dye solution it is fired again for 30 minutes at30°-550° C. (preferably 450-550° C.) in air. Similarly to theelectro-deposition from aqueous Ti(III) solution, this procedure resultsin the deposition of nano-sized TiO₂ particles on the TiO₂ film furtherincreasing its active surface area. Furthermore, this treatment as wellas the anodic deposition of TiO₂ from Ti(III) solution described aboveappears to lead to deposits having a very low impurity content. This iscorroborated by the fact that the treatment becomes ineffective if theTiCl₄ solution is evaporated before firing instead of being washed off.Impurities in the TiCl₄, such as Fe³⁺, are not deposited by hydrolysisfrom the acidic TiCl₄ solution due to the higher solubility of ironoxide compared to TiO₂. By contrast, the evaporation of the TiCl₄solution results in the deposition of impurities. The P25 powdercontains up to 100 ppm Fe₂ O₃, which is known to interefere withelectro-injection of the excited dye. The TiC₄ treatment covers thisrather impure core with a thin layer of highly pure TiO₂ improving theinjection efficiency and the blocking character of thesemi-conductor-electrolyte junction.

Coating of the TiO₂ surface with dyestuff is carried out by soaking thefilm for ca 3h in a 3×10⁻⁴ M solution of the ruthenium complex fromExample 1 in dry ethanol (or alcohol/dimethylsulfoxide mixture). The dyecoating is done immediately after the high temperature annealing inorder to avoid rehydroxylation of the TiO₂ surface or capillarycondensation of water vapor from ambient air inside the nanopores of thefilm. The presence of water in the pores decreases the injectionefficiency of the dye. The electrode is dipped into the dye solutionwhile it is was still hot, i.e. its temperature is ca 80° C. Aftercompletion of the dye adsorption, the electrode is withdrawn from thesolution under a stream of dry air or argon. It is stored in dry ethanolor immediately wetted with the Lil/ILil₃ acetonitrile redox electrolytefor testing. The amount of adsorbed dye is determined by desorbing thedye from the TiO₂ surface into a solution of 10⁻⁴ NaOH in ethanol (oralcohol/dimethylsulfoxide mixture) and measureing its adsorptionspectrum.

A BAS-100 electrochemical analyzer (Bioanalytical Systems, USA) is usedto perform cyclic voltammetry in electrochemical cells of columes of 5to 20 ml. A three-electrode cell is made up of a glassy carbon or Ptdisc (3mm diameter embedded in Teflon) working electrode and a platinumwire counter electrode. The reference electrode consists of calomel incontact with 0.1 LiCl in methanol. It is separated from the workingelectrode compartment by bridge containing the same electrolyte as thetest solution, i.e. 0.1M n-tetrabutylammonium perchlorate inacetonitrile or ethanol. All potentials indicated refer to the aqeueousSCE electrode.

Photo-electrochemical experiments are employed a similar cell equippedwith a flat pyrex window. Alternatively, the dye sensitized TiO₂ film isincorporated into a thin layer sandwich type solar cell. A lightreflecting counterelectrode is employed consisting of a conducting glasssupport onto which a 2 μm thick Pt mirror had been deposited bysputtering. The counterelectrode is placed directly on top of the dyecoated transparent TiO₂ film supported by the conducting glass sheet.Both electrodes are clamped tightly together. A thin layer ofelectrolyte is attracted into the inter-electrode space by capillaryforces. The dyed coated TiO₂ film is illuminated through the conductingglass support. The conversion efficiencies reported are overall yieldswhich are uncorrected for losses due to light absorption and reflectionby the conducting glass support. An Oriel 450 W Xe lamp served as alight source in conjunction with a polycarbonate filter to removeultraviolet radiation and a Schott a 113 interference filter to mimic AM1.5-type solar emission.

The emission spectra are measured on a Spex Ruorolog II equipped with a450 W Xenon light source. The measured excitation and emission spectraare routinely corrected for the wavelength-dependent features usingcorrection factors generated by a National Bureau of Standards 150 Whalogen lamp. The emission detector is a Hamamatsu R2658 photomultiplierwhich extends the corrected emission measurement over a region from 250to 1000 nm. All solutions are prepared by dissolving the appropriateamount of complex in the desired solvent to give typically a 2×10⁻⁵ Msolution. The solutions are degassed by freeze, pump and thaw methods.Low temperature measurements are carried out in an Oxford Instrumentscryostat.

The emission lifetimes are measured by exciting the sample with activemodelocked Nd YAG laser pulse, using the frequency doubled line at 532nm. The emission decay is followed on a Tektronix DSA 602 A DigitizingSignal Analyzer. The digitized xy data is subsequently analyzed andfitted to an exponential model. Sample concentrations were typically1×10⁻⁴ M. Optical densities are taken from spectra recorded on a Cary 5spectrophotometer.

Application Example E

A photovoltaic device shown in FIG. 1 based on the sensitization of atransparent TiO₂ film made from colloidal titanium dioxide particleswhich are deposited on a conducting glass support and sintered to yielda coherent highly porous semiconducting film.

Colloidal titanium oxide particles of approximately 8 nm are prepared byhydrolysis of titanium isopropoxide as follows:

125 ml titanium isopropoxide is added to a solution of 0.1M nitric acidin 750 ml water while stirring. A precipitate of amorphous titaniumdioxide is formed under these conditions. This is heated to 80° C. forapproximately 8 hours, stirring vigorously, resulting in peptisation ofthe precipitate and formation of a clear solution of colloidal anatase.The propanol formed by the hydrolysis is allowed to evaporate during theheating. The colloidal solution is then autoclaved at 230° to 240° C.,in a pressure vessel of titanium metal or teflon for 2 to 20 hours,preferably 16 hours. The resultant sol, containing some precipitate isstirred or shaken to resuspend the precipitate. The resulting sol, minusany precipitate that will not resuspend, is concentrated by evaporationof the solvent in vacuum at room temperature until a viscous liquid isobtained containing the colloidal particles. A typical concentration atthis point is 200 g/L. At this stage a polyethylene oxide polymer, forexample Union Carbide Carbowax 20M or Triton X-405 can be added toincrease the thickness of the layer that be deposited without cracks.The polymer is added in amount of 30 to 50, preferably 40, weightpercent TiO₂.

The electrodes for sensitization are formed from the colloidal solutionas follows:

A suitable substrate, for example a 3×6 cm piece of conductive tin oxidecoated glass, for example from Asahi Corp. (but also titanium metal orany flat conductive surface), is placed with the conductive surface upand with suitable spacers, for example 50 to 100 micron, preferably 80micron thick plastic tape, placed along each edge. A suitable amount ofthe sol, for example 150 microliters of sol with 200 g/L TiO₂ and 40%Carbowax 20M for the above substrate, is pipetted along one end of thesubstrate. The sol is spread across the substrate by drawing with a flatedged piece of glass whose ends ride along the spacers. Thus thespacers, the viscosity of the sol, and the concentration of the solcontrol the amount of TiO₂ deposited. The as spread film is allowed todry in room air till visibly dry and preferable and additional 20minutes. After drying the electrode is fired at 400° to 500° C.,preferably 450, for a minimum of 20 minutes. In the case of solsautocraved below 170° C. the spacers less than 40 micron must be usedand the process must be repeated twice to achieve an 8 to 10 micronthick TiO₂ film.

Electrodes of up to 10 cm by 10 cm have been fabricated by this method.The sol can also be applied to substrates by spin coating and dipcoating.

The electrode can then be cut to the size desired by normal glasscutting techniques. Immediately before applying the sensitizer theelectrode is fired again at 450 to 550, preferably 500° C. for 2 to 12,preferably 6 hours. For some solvent and dye combinations the surface ofthe electrode is improved (with respect to electron injection ) byfiring the electrode 5 to 10, preferably 7 times at 500° C. for 2 to 6hours with either 10 hours in air or soaking up to 1 hour in water, 0.5Mnitric acid or 0.5M HCl, between each firing. The acid solutions aresaturated with dissolved TiO₂ before use. After the last firing,immediately after cooling, the electrode is placed in the sensitizersolution. Preferably an ethanolic (or alcohol/dimethylsulfoxide mixture)solution containing the complex of Example 1 is made up. Depending onthe sensitizer between 4 and 24 hours are required for the electrode togain full color. Full color can be estimated by eye or by taking visiblelight transmittance spectra of the dye at various time.

After removal form the dye solution, the electrode is made into aphotovoltaic cell as follows:

Transparent counterelectrodes are made of the same type of ASAHIconducting glass as the working electrodes. The counterelectrode is notcoated with TiO₂. Instead the equivalent of 10 monolayers of Pt iselectrochemically deposited onto conducting glass. The transparentnature of the counterelectrode is not affected by the deposition of thePt, its transmissions is visible and near infrared remains greater that60%. The Pt acts as an electrocatalyst enhancing the rate of reductionof the electron transfer mediator, i.e. triiodide, at thecounterelectrode. Alternatively, a thin titanium sheet, which may beporous coated as above with Pt, may be used as a counter electrode. Inthe case of a porous sheet, another sheet of impervious material isrequired behind the counter electrode, such as plastic. glass or metal.

A reservoir is provided for the electrolyte by engraving two ca 1 mmdeep and 1.5 mm wide and 20 mm long clefts into the surface of thecounterelectrode close to the edges of the glass sheet. The reservoircan also be added external to the glass sheets or be behind the counterelectrode in the case of porous counter electrode.

The countereletrode is place directly on top of the working electrode toyield a sandwich type configuration. The reservoirs are filled withelectrolyte solution, selected from the list above buy preferably 85% byweight ethylene carbonate, 15% propylene carbonate 0.5M potassium iodideand 40 mM iodine. An amount of Lil or tetraalkylammonium iodide can bepresent (preferably 20 mM) depending on the voltage desire. The cell issealed around the edge with a sealant compatible with the solvent chosenand bonded closed with an adhesive. The sealant and the adhesive may bethe same material for example silicon adhesive in the case of thealcohol solvents, or for example,polyethylene and epoxy resin (ormechanical closure) in the case of ethylene carbonate. The wetting ofthe space between the two electrodes by the electrolyte injected intothe reservoirs occurs spontaneously by capillary action.

Application Examples A to E can be repeated using instead to thecompound of Example 2, the same amount of the 4,4-dicarboxy- or4,4-dialkyl-bipyridyl ruthenium dyes of Examples 1 and 3 to 19.

We claim:
 1. A compound of formula I

    (X).sub.n Ru LL.sub.1                                      (I)

where n is 1 or 2and Ru is ruthenium; each X independently is selectedfrom Cl, SCN, H₂ O, Br, I, CN, --NCO and SeCN;and L is a ligand of theformula ##STR8## and L₁ is selected from a ligand of formula a) to g)##STR9## where each R independently is selected from OH, hydrogen, C₁₋₂₀alkyl, --OR_(a) or --N(R_(a))₂ and each R_(a) independently is hydrogenor C₁₋₄ alkyl.
 2. A compound according to claim 1 in which X is X' whereX' is --Cl, CN, --NCO or --SCN.
 3. A compound according to claim 1 inwhich L₁ is L₁ ' where L₁ ' is selected from a ligand of formula a', b,and c' ##STR10## in which R' is OH, hydrogen or C₁₋₂₀ alkyl.
 4. Acompound of formula II

    (X').sub.2 Ru(L").sub.2                                    (II)

in which L" is a group of formula a" or b ##STR11## and X' is selectedfrom Cl, NCO, CN and SCN; and R" is hydrogen or C₁₋₂₀ alkyl.
 5. Acompound according to claim 2 in which L₁ is L₁ ' where L₁ ' is selectedfrom a ligand of formula a', b, and c' ##STR12## in which R' is OH,hydrogen or C₁₋₂₀ alkyl.
 6. A compound according to claim wherein saidcompound is selected from cis ortrans-dithiocyanato-bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium andcis-dithiocyanato-bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium.